Method of treating iron or mild steel to promote the adherence of porcelain enamel, and stock so produced



July

F. G. SUTPHEN ET AL METHOD OF TREATING IRON OR MILD STEEL TO PROMOTE THEADHERENCE OF PORCELAIN ENAMEL, AND STOCK SO PRODUCED Fi led Sept. 7,1955 INVENTORS. /7?Ea G. 31111-7015, Jzsss J CmvF/zLa,

Foes" L. Mrfes, ties-5P E. 34016 QZIQMM ATTORNEYS.

United States Patent METHOD oF TREATING moN OR- MILD STEEL TO PROMOTETHE ADHERENCE or ro 'cE- LAIN ENAMEL, AND sTocK so PRODUCED Fred G.Sutphen, Jesse J. Canficld, Robert L. Myers, Joseph E. Sams, and RobertS. Burns, Middietown, Ohio, assignors to Armco Steel Corporation,Middletown, Ohio, a corporation of Ohio Application September 7, 1955,Serial No. 532,986

22 Claims. (Cl. 148-16) This application is a continuation-in-part ofour 00- pending application of the same title, Serial No. 101,464, filedJune 25, 1949, since abandoned.

A primary object of our invention is the provision of a process oftreatment for promoting the adherence of porcelain enamels to ferrousmetals. Certain of the teachings herein are applicable to ferrous metalarticles of any kind suitable for porcelain enameling; and in thisconnection it is an object of our invention to provide an adhesionpromoting treatment which is simple, corivenient, and inexpensive.

It is a fundamental object of our invention to provide an improvedenameling. stock made of iron or mild steel in sheet form, and havingthe property of accepting single, fired-on porcelain enamel coatings ofwhite or light color without surface defects and with great improvementin adhesion.

There has in recent years been considerable activity in endeavors tomanufacture iron or mild steel sheet stocks capable of being enameledwith a single, light colored or white coat; but these efforts have beenconcerned principally with the prevention of surface defects. It isessential with such stocks that the metal to be enameled be treated inorder to prevent boiling in the initial coat of porcelain enamel appliedand fired on the surfaces of the metal. Various suggestions have beenmade for such treatments, including the fixing of the carbon in the ironor mild steel sheet stocks by means of titanium or other metallicelements. Iron or mild steel stock may be rendered capable of acceptinga one white coat enamel directly on the base metal with the surface ofthe enamel substantially free from pits and black specks by theinclusion of a surface decarburizing step in the routing-s for theproduction of the stock, and by the correlation of this step with otherprocessing steps whereby to reduce greatly the carbon and othergas-forming substances in and on the sheet surfaces, which substanceshave been found responsible for the surface imperfections appearing inan initial coating of porcelain enamel. The adhesion promoting aspect ofthis application is of importance in the preparation of any of thesestocks inasmuch as, while. the treatments mentioned in varying degreesalleviate the boiling of initial enamel coatings, the problem ofadherence in one coat enameling is of great importance because the useof an undercoat of adhesion promoting enamel is limited or eliminated.As a consequence, another object of this invention is the provision of aprocess especially adaptable to the production of improved one coatWhite enameling stocks, as well as the provision of improved stocks ofthis character. In particular, it is anobject of the invention toprovide an enameling stock not only capable of receiving a single-firedcoat of'white or light colored enamel with substantial or completeelimi- Whenever we refer to white or light colored enamel H in thisspecification, it is to be understood that we include taillight coloredenamels which are the most sensitive to 5 2,755,210 Patented July 17,1956 Ind pits and black speck defects, and which must, therefore, befree from such defects for the sake of appearance, and which are toolight to permit the inclusion of sufiicient cobalt compounds to resultin good adherence without unduly affecting their color.

These and other objects of our invention, which will be set forthhereinafter or will be apparent to one skilled in the art upon readingthese specifications, we accomplish by that procedure and in thatenameling stock of which we shall now describe an exemplary embodiment.

Reference is made to the accompanying drawings wherein:

Fig. l is a cross-sectional photograph at 1000 magnification of ordinaryenameling stock which in its processing included cold reduction on aPangborn or shot blasted rel-l, normalizing, and pickling.

Fig. 2 is a similar view in cross-section of material made in accordancewith this invention.

Fig. 3 is a photomicrograph of the surface of the enameling stock ofFig. 1 under dark field illumination at 500 magnification.

Fig. 4 is a similar view of the surface of a material treated inaccordance with this invention.

T he need for adhesion promoting treatments has hitherto beenrecognized, in view of the experience that the bond of the enamel to thebase metal appeared generally to be the poorest when the enamel surfacewas most perfeet. The deposition of some nickel by chemical displacementupon the surface of an enameling stock has been recognized as atreatment to promote the adhesion of porcelainenamels chemically. Aroughening of the surfaces of enameling stock by various means has alsobeen recognized as promoting adhesion; but the processes hithertosuggested for the obtaining of a useful type of surface roughening havebeen expensive, inconvenient, ineffective, or otherwise undesirable.

We have now discovered a process for the production of channeling stockwhich will have an excellent enamel bond but which will still have allof the surface characteristics of the original material inclusive ofthose materials which have been treated to prevent boiling upon initialfiring.

in the practice of our invention, the routings for the enameling stockinclude, among others, the following steps which must be practiced inthe order mentioned:

1. The surface of the ferrous metal sheet stock or article is coatedwith a thin layer of cobalt or nickel, as will presently be described.

2. The treated stock is scaled, i. e. oxidized, by heating it in anatmosphere oxiding and non-carburizing to iron. This appears to alloy atleast a part of the coating metal with the iron, and produces otherefiects hereinafter discussed.

3. The scaling is followed by an acid pickle. ordinary picklingsolutions may be employed.

We do not desire to be bound by theory, but we believe the enhancedadhesion which is the result of our process is due in part at least tothe production on the surfaces of the iron or steel stocks or articlesof a particular type of roughening which is especially suitable for theretention of enamel. We believe that the coating of imposed metalaffects the nature of the surface oxidation during the scaling operationin an unusual manner, such that when the oxide is pickled from thesurfaces of the stock an especially useful type of roughening occurs. Webelieve that during the oxiding heat treatment, which is step 2indicated above, some alloying of the imposed metal with the base metaloccurs, resulting in unequal distribution of the oxide and the imposedmetal. Upon subsequent pickling, as the scale is removed, the picklingsolution not only readily attacks the surfaces of the metal sheets orarticles, but attacks these surfaces unequally The and preferentially incertain areas, resulting in the particular type of roughening to whichreference has been made.

We shall now discuss the requirements respecting the three steps setforth above before taking up general routings, including these steps,for enameling stocks.

The metal for imposition upon the ferrous enameling stock should becobalt, nickel, mixtures or alloys of the two, or alloys rich in cobalt,nickel or both. Our investigations have shown that cobalt and nickelact, under the conditions of the procedure herein set forth, 111 aunique way toward iron in the surfaces of ferrous enameling stock, inthat minute, adjacent surface areas are formed which differ in theirability to become oxidized, so that, when the oxide is ultimatelyremoved by pickling, a wholly novel type of surface is produced. Othermetals differ as to their ability to alloy with iron, or their abilityto oxidize under processing conditions, or form oxides which sublimeunder operating conditions, or as to their cost and availability. Nickeland cobalt act similarly; they may be used alone or in admixture; andinasmuch as the bare presence of other metallic elements is notdetrimental per se, metallic combinations rich in nickel or cobalt orboth may be used.

As to the quantity of nickel or cobalt applied in the first step above,in general, we apply from substantially .02 to .2 gram per square footof surface of the sheet stock or articles, our preference being for .05to .10 gram per square foot, as producing the most consistent anddependable results.

Nor are we limited to the manner in which the imposed metal is applied.It may be applied by electrochemical displacement, as for example when acleaned ferrous sheet is immersed in a nickel or cobalt salt solutionfor a period of time to permit nickel or cobalt from the bath todisplace iron in the surfaces of the stock or articles. Thus we mayplace a ferrous sheet in a water solution of nickel sulphate containingfrom one-quarter to two ounces per gallon of the salt, or even more. Thebath is preferably maintained at a temperature between 130 and 190 F.;and the sheet is allowed to remain immersed in the salt solution untilthe desired quantity of nickel is deposited upon its surfaces.

Again, the metal may be deposited upon the surfaces of the iron or mildsteel stock or articles by reduction of a salt or salt mixture thereon.For example, as generally set forth in United States Patent 2,101,950 toMcGohan, we may pass ferrous sheets through a substantially 30% to 40%solution of nickel sulphate and permit the nickel sulphate solution todry on the surfaces of the sheet. Thereafter the sheet is subjected to aheat treatment, which results in the decomposition of the nickel salt,and the deposition of nickel on or in the surfaces of the metal. If thisheat treatment is carried out in a neutral or reducing atmosphere as theMcGohan patent prefers, a subsequent scaling anneal must be added. Orthe heat treatment required for the decomposition of the salt may becombined with the scale anneal in our process. A cobalt salt may besimilarly imposed and decomposed.

We prefer to coat the surfaces of the iron or steel stock or articleswith the imposed metal or metals by electroplating, since this isfaster, and the nature of the coating is more easily controlled. Forthis purpose any commercial electroplating bath will serve, for example,the well-known nickel-cyanide or nickel-ammonium-cyanide baths. Ourpreference is for a so-called Watts bath, slightly modified by weakeningthe solution, when working with nickel. The electrolyte in this instancecomprises nickel sulphate and nickel chloride in water solution. Nickelelectrodes are employed, and plating is done at commercial speed with acurrent density of substantially 20 to 30 amperes per square foot. Theplating is, of course, continued only long enough to deposit thequantity set forth above, and it may be noted that physical propertiesof the deposit are not of importanoe in view of the subsequenttreatments to which the stock or article will be put. Hence, otherbaths, including those operating at much higher current densities, maybe employed. Similar procedures are known for the deposition of cobalt.

As to the scale anneal, we have discovered as indicated above that whenenameling stock is coated with the imposed metal or metals within theranges set forth above and then is properly scaled, the result ofpickling in ordinary pickling solutions will be the production of adesirable type of roughness leading, among other things, to greatlyenhanced adherence.

A properly scaled material usually has a characteristic appearance underthe high power microscope after the metal deposition and scale anneal,but before pickling. Beneath the layer of scale there lies a thin,broken layer of iron-nickel alloy, iron-cobalt alloy, or both. Theunusual feature is the appearance of fingers of alloy or oxide whichextend for a considerable distance down into the iron, in some instancesat the grain boundaries, but more often at other points within thegrain. Our inves tigations have indicated that there tends to be moregrain boundary penetration at higher scaling temperatures. We are notcertain whether these fingers are of oxide or alloy of the imposed metalor metals. They do not form in a neutral or reducing atmosphere. Oneexplanation which has been advanced is that the fingers are composed ofoxide which has penetrated the thin imposed metal layer at breaks whichare observed to occur frequently. Subsequently pickling penetrates thesecrevices rapidly, providing much undercutting and irregularity.

In any event, a new type of surface is formed after the pickling. Fig. 1is a cross-sectional view at 1000 magnification of ordinary ironenameling stock showing the nature of its surface. The upper, lighterarea is a sectional view of a copper body lying next the iron. Thiscopper body is in two parts, a mass of copper plated directly againstthe iron surface and an overlying sheet of copper clamped thereagainstand polished with the sample. Certain black specks in the lighter areaindicate the line of demarcation between the copper plating and theoverlying copper sheet. The sample is polished but not etched. Thedarker gray area indicates the iron; and it will be noted that itssurface is characterized by some roughening but substantially nounder-cutting. The sample illustrated in Fig. 1 is characteristic ofordinary enameling stocks, i. e. those which have been cold reduced withthe aid of a Pangborn or shot blasted roll, which in itself producessome surface roughening. The sample has been subjected to a normalizingtreatment and to a pickle. Various routings are common for ordinaryenameling stocks. All such stocks are regularly normalized, but some maybe box annealed following the normalizing treatment, while others may betemper rolled and shipped in that condition. However, the surfaceillustrated in Fig. 1 is characteristic of all of these stocks.

The product illustrated in Fig. 1 has not been subjected to a nickelflash or treatment. While a nickel flash is ordinarily used with theregular enameling stocks, it is a treatment applied in most instances bythe enameler subsequent to forming and pickling and just before thestock or article is enameled.

Fig. 3 is a surface view of the material of Fig. 1. The surface exhibitssome roughening, and will be recalled that it is a pickled surface.There are a few etch pits, presumably along the crystallographicboundaries. But it will be noted that substantially all parts of thesurface are in focus; there is substantially no undercutting; and thesurface might be described as existing in a series of irregular plateausat very slightly different levels.

By way of contrast, reference is made to Fig. 4 wherein a very high andirregular degree of roughening is apparent. In this instance themicroscope was focused at the top levels of the protuberances. The darkareas are indicative of depressions. To bring these into sharp focus,the microscope must be racked considerably, indicating that there is agreat difference in the levels of the tops of the protuberances and thebottoms of the depressions. Furthermore, the photomicrograph of thesurface of materials treated in accordance with this invention exhibitsa characteristic Woolly appearance indicative of the fact that theprojections appear to be spongy, and the protu-berances exist in theform of needles, spicules or tentacles.

This will be further confirmed by reference to Fig. 2 which is across-sectional view of a typical surface of the material of ourinvention at 1000 times magnification. Again, the light area at the topof. the view is. a crosssection of copper, while the darker material isthe iron. But it will be noted that not only is the main surface of theiron very rough, but that above this roughness there are what might betermed floatingv islands of iron which are portions of needles ortentacles having their bases elsewhere than at the specificcross-section. In some instances it will be noted that needle-likeportions of the base metal extend upwardly. Fig. 2 is illustrative ofthe extraordinary degree of undercutting and a spongy type of surfacewhich provides the unsual adherence for vitreous enamel attained in ourprocess.

In our reference to floating islands of iron above, we do not wish to belimited to the exact substances involved, since the tentacles or needlesmay be formed of nickel-iron alloy, cobalt-iron alloy, nickel, cobalt,or combinations of iron and the imposed metal or metals with variousoxides.

The formation of the unusual surface condition shown in Figs. 2 and 4 isdependent upon the presence of nickel, cobalt, or both, and the natureof the scaling treatment to which the material is subjected. Asindicated, the atmosphere throughout the scaling. furnace, to which theenameling stock is subjected, must be oxidizing to iron. We haveattained successful results with a variety of at mospheres includingair, DX gas, burned natural gas with variable ratios of gas to air, andsuch atmospheres in conjunction with varying amounts of moisture. It isknown that moisture affects the oxidizing characteristics ofatmospheres, and it is quite possible toproduce asatisfactory scaling inan atmosphere of steam alone, or in a moisture-containing atmosphere ofa gas otherwise having no. oxidizing characteristics, e, g. nitrogen.Entirely satisfactory results have been secured indry air atmospheres,on the other hand, such as those in the ordinary enameling furnace,although incidental advantages may sometimes be obtained with a highmoisture content, e. g. from 20% to 80% by volume, or by producing. theoxidation by or in the presence of carbon dioxide.

The exact amount of scale produced is not a necessary limitation on theinvention. Further, different chemical compositions of scale areproduced under different conditions. In an endeavor to set forth what webelieve to be about the minimum scaling conditions which will produceour result in combination with the other steps set forth above, we maysay that heating the enamel stock in an atmosphere of burned natural gascontaining 11% combustibles based on dry gas analysis and having a: dewpoint of 150 F. for 2 min. in afurnace at 1 550 P. will give a scalewhich is operative in our process. A more oxidizingv atmosphere orhigher ternperatures may be employed with advantage. For example, anequal or better enamel bond results if the scaling is carried on at asomewhat higher temperature or for a longer time, or both; or if a lowertemperature is used with a lengthened time. Thus, good results areobtained in- 5- min. in a furnace at 1400" F. in the same type of burnedgas atmosphere. A temperature range of l500 to 1-800" F, is preferred byus, although higher temperatures may be employed such: as those upto2000F.,- the top limit being that at which undue grain growth forsatisfactory fabricating properties is encountered.

There is some indication that somuch scale can be put on the stock thatour surface elfect is impaired, althoughv in general the upper limit ofscale is usually determined by pickling time and metal loss. Scalingoccurs very rapidly in air atmosphere, for which reason we usuallyprefer to use a less oxidizing atmosphere, since with it the scale canbe more readily controlled, while the times are not unduly long.Improved results can be attained when scaling in an air atmosphere byapplying the nickel or cobalt in the form of sulphate in such fashionthat there is sulphate remaining on the sheet surfaces at the time ofscaling in air. With atmospheres less oxidizing than air, there islittle likelihood that too much scale will be produced within thecommercial time limitations of an open, i. e. continuous, anneal.

For most purposes a uniform condition of all surface areas of the stockis desired. Hence, the imposition of metal, the scaling treatment, andthe pickle should generally be uniform as respects all surface areas. Anopen or continuous scaling treatment or scale anneal, as contemplated byus, is thus preferably one in which the stock being treated is sentcontinuously through a furnace containing the desired atmosphere withboth sides of the stock open to the atmosphere of the furnace.

The amount and kind of scale preferred are best considered in connectionwith a specific set of conditions which may be used as a referencestandard. If ZO-g'auge enameling sheet (.0359 in. in thickness) isheated to l700 F. in 45 seconds, is held between 1700 and 1800 F. for 40seconds, and is cooled to 900 F. in 2 minutes, the complete cycle beingin an oxidizing atmosphere of burned natural gas, analyzing at thefurnace exit: 9% of carbon dioxide, 5% of carbon monoxide, 12% ofhydrogen, the balance being substantially nitrogen, based on dry gasanalysis and including Water vapor to yield a dew point of to 180 F., avery' desirable scale will be formed. The desirability of scale formedat other temperatures, for different lengths of time, and in otheratmospheres may be gauged by reference to this as a standard.

It will be seen that the attainment of the proper scale is consistentwith commercially economical continuous furnace operation, and thatpermissible temperatures are consistent with normalizing practice ifnormalizing be desired at the stage of the scale anneal ina routing forenameling stock.

The pickling step can be carried on with ordinary acid picklingsolutions, and these we prefer to use for reasons of economy andconvenience. For example, but without limitation, we ordinarily employ asolution of sulphuric acid in Water of a strength ranging substantiallybetween 6% and 25%. Accelerators may be added as. is known in the art,but are not necessary. Such a pickling, solution may be used at atemperature lying substantially between 160 and F. or higher. This isnot limiting, and is indicative of the fact that ordinary picklingsolutions may be employed in Ways entirely consonant with commercialpractice.

Our preference is for sulphuric acid as a pickling agent. Other acids,such as hydrochloric, nitric or phosphoric, may be either too slow inoperation or have varying rates of attack upon the nickel, productive ofless desirable surfaces. However, it is to be noted that once ourdesirable surface is formed, as illustrated in Figs. 2 and 4, anenameling shop picklingv may later by practiced by the enameler in hisplant, Without destroying the desirable surface already formed inaccordance with our invention.

Within the ranges set forth above, the amount of I nickel or cobaltdeposited upon the enameling stock proheavy that it has a tendency topeel away from the surface during pickling. The nature of the scale isimportant in obtaining our results; and differences in the thickness orquantity of the scale will, of course, to some extent influence thepickling time required to remove it. But we have ascertained that wherean enameling stock has been coated with sufiicient nickel or cobalt andproperly scaled as set forth above, a pickling treatment suitable forremoving the scale will at the same time produce the desirable type ofroughening of the metal surface. The pickling time may be increasedwithin very substantial limits without detriment to the product, and insome in stances to its advantage; although it will be obvious that ifthe pickling after the removal of the scale is so prolonged as todestroy entirely the effects produced by the combination of the imposedmetal and the scale anneal, the adhesion produced by our procedure willnot be obtained. In general, the removal of the scale is a reliableguide to the extent of the pickling treatment required. In ourcommercial work, pickling times with sulphuric acid pickling baths rangegenerally from about 1 to about minutes, the preferred range being about2 to about 7 minutes, with materials subjected to the exemplarytreatment above, depending upon the temperature of the bath and thenature of the available pickling equipment.

The steps which we have thus far described produce adherence through theparticular type of roughening action referred to without producingenamel defects. Actually, the process results in the loss of some of theimposed metal which is apparently pickled away during the rougheningtreament. Hence, the function of the imposed metal at this point in theprocess is primarily that of a material which, deposited upon or alloyedwith the surface of the iron or steel stock or articles, the stock orarticles being thereafter suitably scale-annealed, will enable ordinarypickling baths to roughen that surface in the way set forth. In normalpractice, the enameler receiving the sheet stock from the steel willwill ordinarily form the desired finished articles from it by cuttingdrawing, spinning, stamping, forming and the like, after which he willclean and pickle the stock and give it a so-called nickel flash (i. c. anickel coating by chemical displacement). If the utmost effect of nickelas an agent in itself for the chemical promotion of adhesion ofporcelain enamels is desired, these steps should not be omitted. Thus,if the three steps outlined above are practiced on already formedarticles, they are preferably followed by a nickel flash to increase thequantity of nickel at the surfaces of the stock to that desired forchemical adhesion. The amount of nickel so applied is preferably withinthe range of .02 to .30 gram per square foot of surface. The solutionsand procedure for applying this final nickel flash are well understoodin the art and are the same as those previously described for making thefirst application of nickel by chemical displacement. Thus, the imposedmetal treatment coacts with the scale anneal and pickle in our processto control the roughening of the surface, while a final nickel flasheffects the adherence of the enamel to the surface in a chemical manner.

We will consider now exemplary general routings for the production ofenameling stock suitable for receiving single fired coats of white orlight colored enamels.

Ferrous enameling stocks, particularly those which have been reduced togauge by cold rolling, will ordinarily require normalizing to produce adesired grain size and physical characteristics, especially where the'stock is intended for drawing. Normalizing, as is well known, comprisesheating the stock to a temperature above A3 temperature followed by arelatively rapid cooling. It is ordinarily carried on in acontinuous-type furnace.

The decarburizing treatment mentioned hereinafter is a brief continuousheat treatment at a temperature from substantially l250 to 1650 F. in anatmosphere oxidizing to carbon but not oxidizing to iron, and containingfrom about 2% to about 30% of water vapor by volume. Various atmospheresmay be employed, including hydrogen, mixtures of hydrogen and othergases which are either inert or in the particular admixture arenon-oxidizing and non-carburizing toward iron at the temperaturesinvolved. Cracked ammonia gas is a source of a hydrogen-bearingatmosphere which may be employed. The decarburization treatment resultsin a lowcring or elimination of carbon from the surface layers of thestock, the carbon being reduced in the surfaces to a value such that itwill not produce boiling upon initial firing of porcelain enamel. It hasbeen found that such treatments result in a decarburization of thesurface layers of the stock to the extent that a subsequent pickling ofnormal character will not leave on the said surfaces a carbonaceouspickling residue. Also, it has been found that the surfaces of the ironor steel enameling stock are sutficiently decarburized so that migrationof carbon to the surfaces does not occur in a detrimental degree uponbox annealing. For some uses a box annealing is desired following thenormalizing treatment. Once the special surface is formed on theenameling stock in accordance with our invention, it is not destroyed orseriously impaired by subsequent forming or drawing operations to whichthe stock may be subjected. Hence, our process is excellently adaptedfor the production of enameling stocks as such, and does not need to bepracticed by the enameler. This represents a substantial economy.

An exemplary routing for the production of white onecoat enameling stockin accordance with our invention may comprise, as steps subsequent tothe reduction of the metal to gauge by cold rolling, the following:

Normalize Pickle Metal deposition Scale anneal Pickle Decarb urize If abox anneal is desired, it may be added to the above routing as a finaltreatment.

A large number of variations of this routing may be made as long as thesteps of metal deposition, scale annealing, and pickling remain in thisorder. The nickel or cobalt may be applied at any stage preceding thescale anneal. Thus, it may be applied, if desired, prior to thenormalizing step, and will not be destroyed thereby. The decarburizingstep cannot serve as the scale anneal; but the scale anneal can followthe decarburizing step, if desired. Thus, another routing would be:

Normalize Pickle Metal deposition Decarburize Scale anneal Pickle In theprocesses of this invention, where a scale anneal follows the metaldeposition, the scale anneal will perform the function of destroyingundesirable chemical residues which would tend to produce gas upon theinitial firing of porcelain enamel. Thus, the deposition of the nickelor cobalt can follow the decarburization if desired. On the other hand,the presence of the imposed metal during decarburization does notprevent that action occurring, so that it is possible to deposit nickelor cobalt on the surfaces of the stock prior to decarburization, andfollow the decarburization with a scale anneal as set forth above.

The scale anneal is not inconsistent with normalizing, and the two stepsmay be combined with substantial economy where this is desired. Anotherexemplary routarse-mo ingin accordance with the present invention may beset forth thus:

Pickle Metal deposition Decarburization Normalize in oxidizingatmosphere Pickle In a variation of this routing, the metal depositionmay be carried out after the decarburization and before the normalizing,and scaling treatment. It may also be pointed out that if in any routingthe normalizing follows the decarburizing treatment, the grain size maybe somewhat larger than when these steps are in the opposite sequence,and the resulting stock may suffer in drawability. This is because ofthe fact that if the carbon. in an. ironor steel sheet is carried to alow value, the result of a subsequent normalizing treatment will begrain growth. This can be combated by decarburizing the stocksuperficially, i. e. by so carrying on the decarburizing treatment thatwhile the carbon in the surface layers is carried to a sufiiciently lowvalue for single-fired white coat enamel usage, the entire thickness of.the stock is not thoroughly decarburized.

Perhaps the simplest routing to produce a one white coat enameling stockin accordance Witlrthis invention Metal deposition Normalize in an--oxidizing atmosphere Pickle Decarburize In this routing the first heattreatment does double duty both as a normalizing treatment and as. thescale anneal.

Other routings wil suggest themselves. In all such routings theessential requirement must be met that the steps of metal deposition,anenaling toformscale, and acid pickling must appear in. this order.Other processing steps may precede, follow, or be interspersed betweenbetween these steps in indifferent order so far asop'erability isconcerned. It the essential steps of our process afiect such: othersteps, the worker will take this into account in arriving at hispreferred procedure. It is, for

example, ordinarily desirable to decarburize a clean sheet,

so that a pickling prior to decarburizing is usually preferred.

Our invention, when practiced as a process for producing sheet stockdesigned for a single white coat enameling work is not limted by thesubsequent use of any particular enamel. Various enamels differ in theiradherence to the base stock, and in their sensitivity to the variouscauses of pits and black spotsand some of them have good adherencealthough they may have certain disadvantages making them less desirablefor the purpose. Titania base enamels are preferred although they arerather expensive. The fact that they can be applied in a very thin coatand still produce good opacity tends to offset the extra cost and yieldadditional advantages. In addition, they are less sensitive to thepresence of minor amounts of pickling residues and other causes of pitsand black spots. Zirconium enamels are cheaper, but must be used in asomewhat heavier coat. Enamels opacified with antimony are cheaperstill, and adhere very well, but have to be put on in a very thick coatto get good opacity. In addition, they do not cover iron oxide specks aswell as do certain other enamels. Molybdenum enamels exhibit goodadherence, but have poor opacity and are expensive. Our procedure,however, will increase the adherence of all of these enamels in a highlyuseful degree; and frequently enables us to use, with excellentadherence, a more desirable form of enamel which may itself be deficientin its normal adherence to the iron base.

Modifications may be made in our invention without 10 departing from thespirit of it. Having thus described our invention in an exemplaryembodiment, what We claim as new and desire to secure by Letters Patentis:

1. A process of increasing the adherence characteristics of anxiron ormild steel stock or article intended for coating with porcelain enamel,which process comprises in the order named the steps of depositing, uponthe surface of the stock or article a material chosen from a classconsisting of nickel, cobalt, mixtures of the two, and alloys rich innickel, cobalt, or both, subjecting the stock or article to a scaleanneal comprising subjecting the stock or article. to heat in anatmosphere noless oxidizing' than an atmosphere of burned natural gas.containing 11% combustibles based on dry gas analysis and having a dewpoint of F., in afurnace at temperatures of substantially 14-00 to 2000F. for a time of substantially 2 to 5 minutes, whereby to produce adis.- tinct scale, and then acid pickling the article at least to theextent of removing the scale from the surfaces thereof.

2. The process claimed in claim 1 in which the said material isdeposited upon the surfaces of the stock or article in an amount lyingsubstantially between .02 and .2 gram per square foot of surface.

3 The process claimed in claim 1 in: which the said material isdeposited upon the surfaces of the stock or article in an amount lyingsubstantially between .05 and .10 gram per square foot of surface;

4. The process claimed in claim 1 in which the said material isdeposited upon the surface of the stock or article in an. amount lyingsubstantially between .05 and .10 gram per square foot of. surface, andwhich the picklingt-reatment is carried on in a solution of sulphuricacid of a strength substantially between 6% and 25% at temperaturesbetween substantially and F.

5. The process claimed in claim 1 in which the said material isdeposited upon. the surfaces of the stock or article in an amount lyingsubstantially between .02 and .2 gram per square foot of surface andinwhich the said material is applied to the surface of the stock orarticle by electrodeposition.

6. The process claimed in claim 1 in which the said material is nickel.

7. The process claimed in claim. l in. which the said material iscobalt.

8. The process claimed in claim 2 in which the said material is nickel.

9. The process claimed in claim 2 in which the said material is cobalt.

10. The process claimed in claim 5 followed by a subsequent depositionof nickel on the surfaces of the treated stock or article preparatory toenameling.

11. The process claimed in 5 including a becarburizing step for reducingthe carbon in and near the surfaces of said stock or article to a degreenon-productive of boiling upon the initial firing of a coating ofporcelain enamel thereon.

12. A ferrous enameling stock having a deeply undercut surface producedby the process of claim 1.

13. A ferrous enameling stock having a deeply undercut surface producedby the process of claim 6.

14. A ferrous enameling stock having a deeply undercut surface producedby the process of claim 7.

15. A ferrous enameling stock having a deeply undercut surface producedby the process of claim 11.

16. In a process of producing an iron or mild steel porcelain enamelingstock capable of accepting a single coating of white or light coloredporcelain enamel substantially free from pits and specks and having ahigh degree of adherence, the steps of depositing upon the stocksubstantially .02 to .2 gram per square foot of surface of a materialchosen from a class consisting of nickel, cobalt, mixtures of the two,and alloys rich in nickel, cobalt, or both, heating the stock to 1700"F. in 45 secfonds, holding it between 1700 and 1800 F. for 40 seconds,and cooling it to 900 F. in 2 minutes, the complete cycle being in anatmosphere of burned natural gas containing 9% of carbon dioxide, carbonmonoxide, 12% hydrogen, the balance being substantially nitrogen, based011 dry gas analysis, and including water vapor to yield a dew point ofsubstantially to 180 F., acid pickling the oxidized stock with sulphuricacid at least until all the oxide has been removed, the above stepsbeing practiced in the order named and in combination with adecarburizing step in indifferent order with respect to the foregoingsteps for reducing the carbon in and near the surfaces of said stock.

17. A process of producing a ferrous porcelain enameling stock whichincludes the following steps in the order named as practiced upon thesheet gauge stock: normalize, pickle, deposit upon the surfaces of thestock a material chosen from a class consisting of nickel, cobalt,mixtures of the two, and alloys rich in cobalt, nickel, or both,decarburize the stock, subject the stock to a scale anneal by heating itin an atmosphere substantially no less oxidizing than an atmosphere ofburned natural gas containing 11% combustibles based on dry gas analysisand having a dew point of F., in a furnace, at temperaturessubstantially between 1500 and 1800 F. for a time of substantially 2 to5 minutes, whereby to form upon the surfaces of said stock a distinctscale, and pickling the stock for a length of time at least sufiicientto remove the scale from the surfaces thereof.

18. The process claimed in claim 17 in which a box anneal follows thescale anneal and pickle.

19. A ferrous enameling stock having a deeply undercut surface producedby the process of claim 18.

20. A process of increasing the adherence characteristics of an iron ormild steel stock or article intended for coating with porcelain enamel,which comprises in the order named the steps of depositing nickel uponthe surfaces of the stock or article by treating such surfaces withnickel sulphate solution and drying some of said solution on thesurfaces of said stock or article so as to leave nickel sulphate as suchthereon, subjecting the stock or article to a scale anneal in air at anelevated temperature to produce a coating of scale thereon which may beremoved in a period of time substantially between one and ten minutes ina solution of sulphuric acid of substantially 6% to 25% strength at atemperature of substantially 12 to 190 F., and subjecting the stock orarticle to a pickling treatment at least to the extent of removing'saidscale, whereby to produce a roughening of the surfaces of said stock orarticle.

21. A process of increasing the adherence characteristics of an iron ormild steel stock or article intended for coating with porcelain enamel,which comprises applying to the previously decarburized stock or articlea nickel sulphate solution, then normalizing at 1800 F. for two minutesin a controlled atmosphere consisting of 9% carbon dioxide, 5% carbonmonoxide, 12% hydrogen, the balance being substantially nitrogendetermined on a dry basis with water vapor present to give a dew pointof 130 F. to F., and finally removing the scale formed in suchnormalizing treatment by means of a sulphuric acid pickle.

22. A process of increasing the adherence characteristics of an iron ormild steel stock or article intended for coating with porcelain enamel,which comprises applying to the previously decarburized stock or articlea solution of a nickel salt, and with the nickel salt thereon subjectingthe stock or article to a scale anneal producing upon its surfaces scaleat least equivalent substantially to that produced by heating the stockor article in an atmosphere of burned natural gas containing 11%combustibles based on dry gas analysis and having a dew point of 150 F.in a furnace at 1550 F. for five minutes, and then acid pickling thearticle at least to the extent of removing the scale from the surfacesthereof, whereby to produce a roughening of the said surfaces.

References Cited in the file of this patent UNITED STATES PATENTS979,931 Cowper-Coles Dec. 27, 1910 1,360,318 Reiser Nov. 30, 19202,032,256 Canfield et al Feb. 25, 1936 2,101,950 McGohan Dec. 14, 19372,127,388 Canfield Aug. 16, 1938 2,187,844 Scharschu Jan. 23, 19402,199,804 Natthes May 7, 1940 2,206,597 Canfield July 2, 1940 2,449,023Thornton Sept. 7, 1948 2,455,331 Eckel Nov. 30, 1948

1. A PROCESS OF INCREASING THE ADHERENCE CHARACTERISTICS OF AN IRON ORMILD STEEL STOCK OR ARTICLE INTENDED FOR COATING WITH PORCELAIN ENAMEL,WHICH PROCESS COMPRISES IN THE ORDER NAMED THE STEPS OF DEPOSITING UPONTHE SURFACE OF THE STOCK OR ARTICLE A MATERIAL CHOSEN FROM A CLASSCONSISTING OF NICKEL, COBALT, MIXTURES OF THE TWO, AND ALLOYS RICH INNICKEL, COBALT, OR BOTH, SUBJECTING THE STOCK OR ARTICLE TO A SCALEANNEAL COMPRISING SUBJECTING THE STOCK OR ARTICLE TO HEAT IN ANATMOSPHERE NO LESS OXIDIZING THAN AN ATMOSPHERE OF BURNED NATURAL GASCONTAINING 11% COMBUSTIBLES BASED ON DRY GAS ANALYSIS AND HAVING A DEWPOINT OF 150* F., IN A FURNACE AT TEMPERATURES OF SUBSTANTIALLY 1400* TO2000* F. FOR A TIME OF SUBSTANTIALLY 2 TO 5 MINUTES, WHEREBY TO PRODUCEA DISTINCT SCALE, AND THEN ACID PICKLING THE ARTICLE AT LEAST TO THEEXTENT OF REMOVING THE SCALE FROM THE SURFACES THEREOF.